1. Field of the Invention
This invention relates to a process for preparing a matte vinyl chloride resin useful as high quality matte molding material owing to minimized fish-eyes and good workability and a process for preparing a matte vinyl chloride polymer composition.
2. Prior Art
In general, plasticized vinyl chloride resins are widely used as wire coatings, hoses, tubes, packings, gaskets, leather sheets and household goods. There is an increasing demand for matte products and in this regard, many conventional vinyl chloride resin products fail to fully satisfy users' demand.
Since vinyl chloride resins are improved in physical properties such as mechanical strength and compression set as their degree of polymerization increases, an appropriate degree of polymerization is selected in accordance with the set of physical properties required in a particular application. Despite such improvement in physical properties, vinyl chloride resins having a higher degree of polymerization are undesirably low in workability.
On the other hand, vinyl chloride-diallyl phthalate copolymers, which are abbreviated as VC-DAP copolymers, hereinafter, are known to be easily workable, have superior physical properties such as mechanical strength, compression set and heat resistance as compared with vinyl chloride copolymers having an identical degree of polymerization, and afford a matte finish outer appearance to molded products.
For example, Japanese Patent Publication (JP-B) No. 27774/1984 discloses a vinyl chloride resin composition comprising 100 parts by weight of a vinyl chloride resin in the form of a VC-DAP suspension copolymer which is fully soluble in tetrahydrofuran (THF) and has an average degree of polymerization of at least 2,000 and 65 to 200 parts by weight of a plasticizer. This vinyl chloride resin composition comprising a plasticized vinyl chloride polymer (VC-DAP copolymer) has the advantage of minimized fish-eyes, but suffers from the problem that when extrusion molded at high temperatures, molded parts have a less matte outer appearance and when molded at low temperatures, the gelling time is long though shorter than vinyl chloride homopolymers.
Japanese Patent Application Kokai (JP-A) No. 117550/1979 discloses a vinyl chloride resin composition comprising 100 parts by weight of a VC-DAP copolymer consisting of 1 to 90% by weight of a THF insoluble gel component and the remainder of a THF soluble component and 25 to 200 parts by weight of a plasticizer. This vinyl chloride resin composition comprising a plasticized VC-DAP copolymer has the problem that when the THF insoluble gel component of the VC-DAP copolymer exceeds 2.5% by weight, molded parts have a fully matte outer appearance, but contain many fish-eyes, and the matte finish of molded parts is rather rough in texture, so that the outer appearance is less attractive. When the THF insoluble gel component of the VC-DAP copolymer is less than 2.5% by weight, molded parts of the vinyl chloride resin composition are satisfactory in that less fish-eyes are contained, the matte finish of molded parts is fine in texture, and the outer appearance gives a satisfactory matte finish. Regrettably, it is difficult to prepare in a reproducible manner a VC-DAP copolymer containing such a limited amount of gel as less than 2.5% by weight of the THF insoluble gel component.
More particularly, when it is desired to prepare a VC-DAP copolymer containing such a limited amount of gel as less than 2.5% by weight of the THF insoluble gel component, the polymerization temperature and the amount of diallyl phthalate added must be carefully controlled. These conditions are in the critical region where the THF insoluble gel component forms. During polymerization under identical conditions, depending on a subtle change of polymerization conditions, the THF insoluble gel component will form in a large amount in some cases, but no gel component will form in other cases. On this account, it is difficult to prepare in a reproducible manner a VC-DAP copolymer containing such a limited amount of gel as less than 2.5% by weight of the THF insoluble gel component.
As the area of application of matte vinyl chloride polymers and compositions thereof is expanded, a severer demand is imposed on the quality of such polymers and compositions. It is desired to minimize fish-eyes especially for film and sheet form applications, and it is desired to have molded parts of quality offering a matte outer appearance whose matte finish is fine in texture and uniform.